1. Field of the Invention
The present invention relates to a base station search method and mobile terminal apparatus in a CDMA-based mobile communication.
2. Description of the Related Art
A CDMA-based mobile communication system includes a plurality of base stations in a service area and a mobile station transmits/receives signals to/from a nearest base station.
A radio zone covered by one base station is called a “cell (or sector)”.
When power is turned on, a mobile station searches for a nearest base station, then transmits/receives predetermined information to/from the searched base station and finally enters into a communication ready state (ready for receiving a call signal from the base station: standby state).
A CDMA system is broadly divided into W-CDMA system (European-Japanese system) and CDMA2000 (IS-95 system: USA system).
According to the first system, each base station operates asynchronously. On the other hand, the second system supplies each base station with an absolute time base using a GPS satellite, etc. so that each base station operates synchronously.
However, a procedure for a mobile station to search for a nearest base station (a method whereby a mobile station searches for a cell to which it belongs, and which is generally called a “cell search”) is basically common to both systems.
That is, in a first stage of a cell search, the mobile station detects signals that reach a minimum reception limit level out of signals (pilot symbols included in a perch channel and long code mask symbols, etc.) sent by each base station to acquire synchronization.
Then, the mobile station selects a signal with maximum reception power out of a plurality of detected signals and then controls the phase of a despreading code in an attempt to establish synchronization with the base station corresponding to the selected signal.
In a second stage of the cell search, the mobile station requests the nearest base station with which synchronization has been established to proceed with position registration and at the same time sends information on the current reception power.
Upon receipt of a request for position registration, the base station newly registers the position of the mobile station in memory and controls the transmit power.
This allows the mobile station to perform transmission/reception with the base station and the mobile station enters into a so-called standby status.
A plurality of base stations is located in a service area in such a way that any part of the service area is covered by radio zones (cells) of any of those base stations.
However, there are actually a wide variety of topographic features and radio wave propagation environments in the regions where mobile communication services are provided and there are regions in the same service area where radio waves from base stations cannot be received with ease.
Moreover, a communication environment changes from moment to moment due to variations of weather, an amount of communication requested to one base station (amount of traffic) or construction of new buildings and so on. Therefore, even in an area where communications could be conducted under normal circumstances, communications may become impossible when unfavorable conditions are combined.
As described above, the mobile station decides whether communication is possible or not based on whether reception power of a perch channel signal (pilot symbol and long code mask symbol) received from the nearest base station reaches a minimum reception limit level or not.
That is, when the reception power of a pilot signal received from the nearest base station does not reach the minimum reception limit level, the mobile station decides that mobile station is located outside the radio zone (cell) covered by the nearest base station and stops sending a position registration request or power control information, etc.
In this condition, the base station (and a base station control apparatus) decides that there is no more mobile station that can communicate with the base station in the cell and deletes the position registration, which makes further communications impossible.
However, there are also a variety of modes in which it is difficult to receive radio waves from the nearest base station and communication is decided to be impossible in the same way as in the aforementioned case and there can also be cases where it is possible to maintain or secure communication by taking some action.
According to conventional technologies, when there is none of a plurality of received signals that exceeds the minimum reception limit level, the mobile station gives up communication of a position registration request across the board, which eliminates the possibility of communicating with the base station.
In a communication using a mobile terminal such as a cellular phone, it is extremely important to allow the user to communicate wherever the user is located.
Therefore, it is useful to allow a mobile radio terminal to access the base station more easily with high reliability even in a region where the electric field of a received signal is weak and where power of the received signal is at a minimum limit level from the standpoint of enhancing convenience of a mobile communication.